This paper investigates the secrecy performance of an underlay cognitive radio network (CRN) with outdated channel state information (CSI) for two practical scenarios, ie, scenario 1: the CSI of an eavesdropper’s channel is unavailable at the secondary transmitter and scenario II: the CSI of an eavesdropper is available at the secondary transmitter. The selection combining (SC) and maximal ratio combining (MRC) techniques are utilized at the secondary receiver while MRC is also employed at both eavesdropper and primary receiver. For the scenario I, the compact expressions of exact and asymptotic secrecy outage probability (SOP), intercept probability and ϵ-outage secrecy capacity are derived, and simulations are conducted to verify analytical results. Our results reveal that the secrecy performance of the system degrades when the CSI of the main channel is outdated. Additionally, in scenario II, a comprehensive analysis of average secrecy capacity is performed, and new closed-form expressions for exact and asymptotic average secrecy capacity are derived which are valid for an arbitrary number of antennas at secondary receiver and eavesdropper. Finally, we investigate the high signal to noise ratio (SNR) slope and high SNR power offset which characterize the impact of the outdated CSI of the main channel and eavesdropper’s channel on average secrecy capacity.